With reduction in size and weight of electronic appliances, electrical parts such as batteries and capacitors used in these appliances are also desired to achieve size and weight reduction. Thus, for example, nonaqueous electrolyte cells that use a bag as a sealable container in which a nonaqueous electrolyte (electrolytic solution), a positive electrode, and a negative electrode are sealed have been employed. An electrolytic solution prepared by dissolving a fluorine-containing lithium salt, such as LiPF6 or LiBF4, in a nonaqueous organic solvent such as propylene carbonate, ethylene carbonate, dimethyl carbonate, or ethylene methyl carbonate, has been used as the nonaqueous electrolyte.
The sealable container is required to prevent penetration of electrolytic solutions and gas and entry of moisture from outside and thus a container having a bag shape formed by heat-sealing laminating films each including a metal layer such as an aluminum foil coated with resin has been used as the sealable container.
One end of the sealable container is formed as an open portion. Lead conductors that are connected to one end of a positive electrode plate and one end of a negative electrode plate sealed in the container are arranged to extend from the inside of the sealable container to the outside of the sealable container through the open portion. The open portion is heat-sealed to bond the sealable container and the lead conductors and to thereby seal the open portion. This portion to be heat-sealed last is referred to as a “seal part”.
In the seal part, the sealable container and the lead conductors are bonded (heat-sealed) by using a thermal adhesive layer. The thermal adhesive layer is formed in advance on the lead conductor in a portion corresponding to the seal part in some cases or on the sealable container in a portion corresponding to the seal part in other cases. In either case, the seal part is required to maintain sealing property (sealing performance) without causing short-circuiting between the metal layer and the lead conductor.
PTL 1 discloses a battery sealing bag and a lead wire for use in such a nonaqueous electrolyte cell and describes that the sealing performance of the seal part can be enhanced by using a thermal adhesive layer formed by forming a maleic acid modified polyolefin layer directly on the conductor of the lead conductor.
However, although the adhesive property immediately after sealing is sufficient, moisture that has penetrated the seal part reacts with the electrolyte sealed inside the sealable container with long-term use, hydrofluoric acid is generated and corrodes the lead conductor (metal), and separation ultimately occurs at the interface with the metal layer or the lead conductor.
PTL 2 discloses a seal part that remains uncorroded despite generation of hydrofluoric acid due to the reaction between the electrolyte and moisture and maintains the adhesive property. In particular, PTL 2 discloses a battery tab constituted by a lead conductor having a nickel surface and a composite coating layer composed of an aminated phenol polymer, a trivalent chromium compound, and a phosphorus compound and covering the surfaces and the side surfaces of the lead conductor. PTL 2 describes that the composite coating layer prevents elution of the nickel layer and corrosion of the nickel layer caused by hydrofluoric acid generated from the electrolyte and moisture.
PTL 3 proposes forming a layer composed of an acid modified styrene-based elastomer in a seal part in a portion that comes into contact with the lead conductor.